Under ordinary circumstances, the thermoregulatory mechanisms of a healthy human body serve to maintain the body at a constant temperature of about 37° C. (98.6° F.), a condition sometimes referred to as “normothermia.” To maintain normothermnia, the thermoregulatory mechanisms act so that heat lost from the person's body is replaced by the same amount of heat generated by metabolic activity within the body. For various reasons such as extreme environmental exposure to a cold environment or loss of thermoregulatory ability as a result of disease or anesthesia, a person may develop a body temperature that is below normal, a condition known as “hypothermia.” A person may develop a condition that is above normothermia, a condition known as “hyperthermia”, as a result of extreme exposure to a hot environment, or malfunctioning thermoregulatory mechanisms, the latter being a condition sometimes called “malignant hyperthermia.” The body may also establish a set point temperature (that is, the temperature which the body's thermoregulatory mechanisms function to maintain) that is above normothermia, a condition usually referred to as “fever.” The present invention addresses these and other situations requiring alteration of at least a portion of a patient's body temperature.
Accidental hypothermia is generally a dangerous condition that may even be life threatening, and requires treatment. If severe, for example where the body temperature drops below 30° C., hypothermia may have serious consequences such as cardiac arrhythmias, inability of the blood to clot normally, or interference with normal metabolism. If the period of hypothermia is extensive, the patient may even experience impaired immune response and increased incidence of infection.
Simple methods for treating accidental hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. If the hypothermia is not too severe, these methods may be effective. However, wrapping a patient in a blanket depends on the ability of the patient's own body to generate heat to re-warm the body. Administering warm fluids by mouth relies on the patient's ability to swallow, and is limited in the temperature of the liquid consumed and the amount of fluid that may be administered in a limited period of time. Immersing a patient in warm water is often impractical, particularly if the patient is simultaneously undergoing surgery or some other medical procedure.
More recently, hypothermia may be treated in a more complex fashion. Heated warming blankets may be applied to a patient or warming lamps that apply heat to the skin of the patient may be used. Heat applied to the patient's skin, however, has to transmit through the skin by conduction or radiation which may be slow and inefficient, and the blood flow to the skin may be shut down by the body's thermoregulatory response, and thus, even if the skin is warmed, such mechanisms may be ineffective in providing heat to the core of the patient's body. When breathing gases are administered to a patient, for example a patient under anesthesia, the breathing gases may be warmed. This provides heat relatively fast to the patient, but the amount of heat that can be administered without injuring the patient's lungs is very limited. An alternative method of warming a hypothermic patient involves infusing a hot liquid into the patient via an IV infusion, but this is limited by the amount of liquid that can be infused and the temperature of the liquid.
In extreme situations, a very invasive method may be employed to control hypothermia. Shunts may be placed into the patient to direct blood from the patient through an external machine such as a cardiopulmonary bypass (CPB) machine which includes a heater. In this way, the blood may be removed from the patient, heated externally, and pumped back into the patient. Such extreme measures have obvious advantages as to effectiveness, but also obvious drawbacks as to invasiveness. The pumping of blood through an external circuit that treats the blood is generally quite damaging to the blood, and the procedure is only possible in a hospital setting with highly trained personnel operating the equipment.
Accidental hyperthermia may also result from various conditions. Where the normal thermoregulatory ability of the body is lost, because of disease or anesthesia, run-away hyperthermia, also known as malignant hyperthermia, may result. The body may also set a higher than normal set point resulting in fever which is a type of hyperthermia. Like hypothermia, accidental hyperthermia is a serious condition that may sometimes be harmful, even fatal. In particular, hyperthermia has been found to be neurodestructive, both in itself or in conjunction with other health problems such as traumatic brain injury or stroke, where a body temperature in excess of normal has been shown to result in dramatically worse outcomes, even death.
As with hypothermia, when the condition is not too severe, simple methods for treating the condition exist, such as cold water baths and cooling blankets, or antipyretic drugs such as aspirin or acetaminophen, and for the more extreme cases, more effective but complex and invasive means such as cooled breathing gases, cold infusions, and blood cooled during CPB also exist. These, however, are subject to the limitations and complications as described above in connection with hypothermia.
Although both hypothermia and hyperthermia may be harmful and require treatment in some case, in other cases hyperthermia, and especially hypothermia, may be therapeutic or otherwise advantageous, and therefore may be intentionally induced. For example, periods of cardiac arrest or cardiac insufficiency in heart surgery result in insufficient blood to the brain and spinal cord, and thus can produce brain damage or other nerve damage.
Hypothermia is recognized in the medical community as an accepted neuroprotectant and therefore a patient is often kept in a state of induced hypothermia. Hypothermia also has similar advantageous protective ability for treating or minimizing the adverse effects of certain neurological diseases or disorders such as head trauma, spinal trauma and hemorrhagic or ischemic stroke. Moreover, hypothermia has been found to be protective of the kidneys from damage due to exposure to nephrotoxic contrast media, such as is used during vasculature imaging methods like coronary angiography.
For the above reasons and others, it is sometimes desirable to induce whole-body or regional hypothermia for the purpose of facilitating or minimizing adverse effects of certain surgical or interventional procedures such as open heart surgery, aneurysm repair surgeries, endovascular aneurysm repair procedures, spinal surgeries, or other surgeries where blood flow to the brain, spinal cord or vital organs may be interrupted or compromised. Hypothermia has also been found to be advantageous to protect cardiac muscle tissue after a myocardial infarct (MI).
Current methods of attempting to induce hypothermia generally involve constant surface cooling, by cooling blanket or by alcohol or ice water rubs. However, such cooling methods are extremely cumbersome, and generally ineffective to cool the body's core. The body's response to alcohol or ice water applied to the surface is to shut down the circulation of blood through the capillary beds, and to the surface of the body generally, and thus to prevent the cold surface from cooling the core. If the surface cooling works at all, it does so very slowly. There is also an inability to precisely control the temperature of the patient by this method. Patient safety issues may arise when, for example, ice water baths are used in the presence of defibrillators and other common hospital equipment.
If the patient is in a surgical setting, the patient may be anesthetized and cooled by cardiopulmonary bypass as described above. Generally, however, this is only available in the most extreme situations involving a full surgical team and full surgical suite, and importantly, is only available for a short period of time because of the damage to the blood caused by pumping the blood through the extracorporeal circuit comprised of pumps and tubing. Generally surgeons do not wish to pump the blood for periods longer than 4 hours, and in the case of stroke or traumatic brain damage, it may be desirable to induce hypothermia for longer than a full day. Because of the direct control of the temperature of a large amount of blood, this method allows fairly precise control of the patient's temperature. However, it is this very external manipulation of large amounts of the patient's blood that makes long term use of this procedure very undesirable.
Means for effectively adding or removing heat to or from the core of the body that do not involve pumping the blood with an external, mechanical pump have been suggested. For example, a method of treating hypothermia or hyperthermia by means of a heat exchange catheter placed in the bloodstream of a patient was described in U.S. Pat. No. 5,486,208 to Ginsburg, the complete disclosure of which is incorporated herein by reference. Means of controlling the temperature of a patient by controlling such a system is disclosed in U.S. Pat. No. 5,837,003, also to Ginsburg, the complete disclosure of which is incorporated herein by reference. A further system for such controlled intervascular temperature control is disclosed in U.S. Pat. No. 6,620,188 to Ginsburg et al., and U.S. Pat. No. 6,849,083 to Ginsburg, the complete disclosure of which is incorporated herein by reference. Those patents and publication disclose a method of treating or inducing hypothermia by inserting a heat exchange catheter having a heat exchange area including a balloon with heat exchange fins into the bloodstream of a patient, and circulating heat exchange fluid through the balloon while the balloon is in contact with the blood to add or remove heat from the bloodstream. (As used herein, a balloon is a structure that is readily inflated under pressure and collapsed under vacuum.)
A number of catheter systems for cooling tissue adjacent the catheter or regulating the temperature of the catheter using the temperature of fluid circulating within the catheter are shown in the published art. Some such catheters rely on a reservoir or similar tank for a supply of heat exchange fluid. For example, U.S. Pat. No. 3,425,419 to Dato, U.S. Pat. No. 5,423,811 to Imran et al., U.S. Pat. No. 5,733,319 to Neilson, et al., and U.S. Pat. No. 6,019,783 to Phillips, et al., disclose catheters with circulating heat exchange fluid from a tank or reservoir. For such systems that involve a catheter placed in the bloodstream, however, difficulties arise in sterilizing the fluid source between uses and rapidly changing the temperature of a large volume of fluid having a significant thermal mass.
It has been recognized that certain situations call for more cooling power than has been available using present systems. For example, it is postulated that reducing the time to cool a patient's blood immediately after a stroke or coronary event may improve the chance that the patient will recover, or at least reduce the amount of damage done due to ischemia. One way to reduce the time necessary to cool a patient's core body temperature to a desired target value is to maximize the cooling power available to remove heat from the patient's blood. However, presently available systems, such as those described above, are limited in the cooling power they can provide, or are too invasive as described in the case of heat exchange with extracorporeal circulation.
For the foregoing reasons, there is a need for a rapid and effective means to add or remove heat from the fluid supply for a catheter used to control the body temperature of a patient in an effective and efficient manner, while avoiding the inadequacies of the prior art methods. Such a system would rapidly, efficiently and controllably provide for heating or cooling the temperature of a patient or target tissue, and regulates the temperature of the patient or target tissue based on feedback from the temperature of the patient or target tissue. It would be particularly advantageous to provide a system that would reliably supply increased cooling power compared to present systems, provide for enhanced removal of heat from a patient's body, decrease the time necessary to reduce the patient's body temperature to a desired target temperature, and that may be deployed in both surgical and general wards of the hospital by at least one operator. The present invention fulfills this and other needs.